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I was once asked at a University PPE interview, Does time have a colour? I found it both extremely interesting and baffling. My opinion was that as time was not a physical property it could not have a colour yet I questioned myself countless times. What's your opinion - could time have a colour?
K(17)

I suspect the point of this question was to see whether you could articulate the idea of a "category error," that is a statement that is syntactically correct but is nonsense because its predicate cannot meaningfully be attributed to its subject.

If this is what the interviewer had in mind, your answer was essentially correct but could have been stronger if you had explained that this was one example of a more general problem. If your interview was at an Oxford college, you probably would have earned bonus points if you had referred to Gilbert Ryle's classic discussion of category errors.

The word 'color' has three meanings, as far as I can tell: 1, certain properties of atoms and molecules that make them emit electromagnetic radiation in the so-called visible range; 2, mixtures of frequencies of this electromagnetic radiation that go to the eye of the observer and produce an image on his/hers retina; and 3, sensations of color that this observer experiences. So if I am looking at a green leaf, which of these three meanings of 'green' am I experiencing?

John Locke makes a similar distinction between primary qualities -- roughly your (1) and (2) -- and secondary qualities -- your (3). On his analysis, this is between qualities that actually inhere in things, and qualities that are only in our ideas of things because they are a result of the relation of a thing and our senses.

It seems to me that your question already answers itself. You define meaning 3 as 'sensations of color that this observer experiences'; but your question reads 'which of these three ... am I experiencing?' Perhaps though you meant to use the word 'experience' in two different senses. The first sense (used in definition 3) would be 'the purely mental content that results from the external influence'; the second sense (used in the question) would be 'what is most immediately encountered, rather than inferred.'

Current common sense would tend to answer in the same way you already did, inadvertently. It is the color sensation that I encounter immediately and thus which I experience.

However, consider the following example. A botanist is looking at a tree and she says 'I see that this specimen is lacking potassium.' Must we assume that she sees a particular shade of green and this is for her a sign that indicates the health of the tree? Or can we argue instead that she straightforwardly experiences a tree lacking potassium, and that it would take a special effort to 'back up' from her professional activity, and 'see' just the color green. Philosophers working in the phenomenological tradition are inclined to understand things in the second way.

Even supposing that we accept that analysis, it doesn't yet answer your question. For only a physicist interested in electromagnetic radiation (or perhaps a philosopher pretending to be a physicist!) could be plausibly said to 'experience' a color in that way. As above, a botanist would instead experience a leaf of this or that species, this or that state of health. An interior decorator would see 'just the right' or 'entirely the wrong' paint. Your question assumes (with good reason, and in agreement with Locke) that physics offers the most fundamental and universal account of the 'primary quality' meaning of colour. But that does not necessarily mean it is the most immediately available and useful account.

So, at the very least, we should add a fourth possibility to your list: 'If I am looking at a green leaf, which 'green' am I experiencing?' (4) the green of the leaf.

Instead of answering your question, I just managed to make it more difficult. Apologies.

I am having trouble with secondary qualities, which are manufactured in the brain after receipt of digital signals from the sense organs. For example, if I see a green leaf, I know that chlorphyll molecules in the leaf transmit electromagnetic radiation of a frequency such as to produce a sensation of green in my brain. The problem is that all the empirical objects that I perceive are structures of secondary qualities, and these are all outside my head. So where are secondary qualities, inside my head, or outside?

I don't know how much this will help, but there seem to be three things associated with the color green: the experience of green inside your head; the disposition of certain surfaces to produce that experience, something which is not inside your head though it is defined in terms of something inside your head; and the molecular structure of certain surfaces out there that help to cause experiences of green. Most philosophers agree that all three of these things exist, but they disagree about which of them should be identified with 'the color green'.

The color of something is the color of the spectrum that isn't taken in by an object. However when I look at the color "green", do I see the same tint someone sees when they see "blue"? The identification of a color is what we've been told, and we've essentially been told what colors don't go good together. So how do we know that all of our eyes see the same thing?
-Samantha B.

This is a classic pr0blem in philosophy, the problem of 'spectrum inversion'. Even if you see blue like I see red, and vice versa, it is very difficult to see how we could ever tell. I cannot see your experiences, and you would use the words 'blue' and 'red' the same way I do, since you were taught to say 'blue' when you saw blue objects and 'red' when you saw red objects, even if your experiences were different. It's interested that spectrum inversion is different from color-blindness. There we can tell, because color blind people can make fewer discriminations.

Suppose that colors have a fourth dimension to which we, humans, are not sensitive. As a matter of fact, I gather that this is true, and that certain birds are now thought to be sensitive to just such an additional dimension. I don't see why this would challenge the existence of the colors we now see. The colors are there, it's just that we're not sensitive to all their aspects.

But this answer assumes a sort of "color objectivism"--that is, that colors are properties out there in the world (certain reflectance patterns perhaps) that obtain independently of our seeing them. Suppose instead that we think of colors more subjectively, as existing in our minds--perhaps as qualitative features of our color-experiences. (I don't think this is the correct way to think of colors, but others do, and certainly some of our color-talk seems to embody this view.) If we think of colors this way (call them "subjective-colors"), then the subjective-colors of the birds--what it's like to have their color-experiences--are quite different from ours. I can't even imagine what it's like to add in sensitivity to that fourth dimension (even after learning a bit about what types of discriminations it allows the birds to make). Perhaps this is like the difference between the experiences of color-blind people and of those who aren't, or perhaps it's more radical. All of this raises interesting questions about how we might (or whether we can) compare experiential qualities across different types of preceptual systems. However, as far as I can see, none of this tells against the existence of our "colors" on this way of speaking--that is, against our subjective-colors. They clearly exist. My computer screen is loaded with objective-colors, and thus my current experience of looking at it is loaded with subjective-colors, even if they are three-dimensional.

The answer to this question obviously depends upon what color is. and I haven't a clue. But there is a little we can say anyway. Let us ask: Could there be a color that human beings perceived that was utterly unlike any other? To this question, I think the answer is no. Human color perception, it is now pretty well-established, is tri-chromatic, which means that color-space, as humans perceive it, is three-dimensional, the three dimensions characterized by the primary colors. I suppose it is possible that there are some colors no one has ever perceived that would appear utterly unlike colors to which they were nonetheless chromatically related, but human color vision also appears to satisfy some kind of continuity principle that would rule out that possibility.

Some birds, it appears, have vision based upon four dimensions. If so, then the structure of color-space as they perceive it is radically unlike ours. Whether that means that there are colors they perceive that we do not, or whether it instead means that they see similarities that we do not, is much debated.

What color is clear? When something is clear, it is whatever color the thing behind it is, but, if you imagine that nothing is behind it, what would it look like? It couldn't be white or black, because that isn't clear, that would be white or black.

How can we be sure that we perceive color the same way? In other words, how do I know that the red I see looks the same as the red that you see?
We are taught from birth to identify red objects as red, but what if what someone calls red really looks green for example, yet they only call it red because that is what has been taught?

This is a natural and important question to wonder about. It is also an old and distinguished one, dating back at least to John Locke. In its modern incarnation it's often called the problem of "spectrum inversion" or "qualia inversion". Two people might make all the same color discriminations, and use color language in all the same ways, so that outwardly (from a third-person perspective) we would have no reason to say that colors don't "look the same way" to them. But how can we be sure? Isn't it possible that a red object looks to me exactly the same way a green object looks to my functional twin, and vice versa?

There is currently a raging disagreement about this, and it leads directly into the fascinating and vexing mind-body problem. Much of the debate turns on what we should mean by "the way a color looks" or "the qualitative character" color experience. Some hold that the way things look or seem first-personally does not go beyond the way that a person reacts to, processes, and acts upon her environment. For such philosophers ("functionalists") if two people really are functionally the same--that is, they make all the same color discriminations and use color words in all the same ways--then things thereby look the same to them from the inside. Philosophers who hold this need somehow to explain away the intuition you expressed--that it's nevertheless possible for things to look differently to such twins. Other philosophers honor the intuition and hold, therefore, that the felt, phenomenal character of a color experience is not exhausted by the role that that color experience plays in a person's cognitive functioning. But then one promising materialist way that we might try to understand our mental states--as states that essentially play a certain cognitive role in us--can't capture everything. In fact, it leaves out an experience's most central aspect--the way it feels from the inside. And if this transcends cognitive function, then perhaps it's not physical at all.

It might also be useful to distinguish the color green from the experience of that colour. Some philosophers (and scientistists, e.g. Galileo) have held that the color just is the experience, but it is m0re common and more plausible to distinguish them. Some would identify the colour with a disposition to produce the experience (which is distinct from experience itself, since it may be present in the dark), some would identify the colour with physical properties of the surfaces of objects, and there are other views as well. Anyway, if colour is say a property of surfaces to reflect light at certain frequencies, then this is something that can be explained to a blind person. But when it comes to the experience itself, it may well be that someone who has not had any visual experience is not in a position to have the full concept of color that the sighted have, and so not in a position to understand the experience as fully as the sighted can.

The answer to this may be yes and no. Yes, colors are or are caused by micro-properties of the surface of objects, but apparently quite different micro-structures may correspond to the same perceived color.